it depends a whole lot on your climate. If your chickens will want to spend their days outdoors virtually all day every day all year round even in stretches of nasty weather, you really just need somewhere for them to roost at night, so 2-3 sq ft per chicken indoors may suffice if necessary. Most BYCers allow 4 sq ft per chicken (e.t.a. - INDOORS! plus larger run or free-range).
Personally I think that's pretty cramped if the chickens will be indoors for long stretches in weather they don't like, and having *tried* the higher stocking densities and seen how differently they act at lower stocking densities, I would not give them less than 10-15 sq ft (indoors) per chicken myself. They wouldn't mind having more than that, I'm sure.
Really you just have to look at what's the maximum size coop you can realistically build, then consult your conscience about how many chickens you want to try to shoehorn in. It's partly a practical matter -- the more crowding, the greater the chance of picking/cannibalism which sometimes can only be 'cured' via making soup and starting over, and also greater crowding makes coop cleaning much mroe of a chore. But it's also partly an ethical matter, as chickens really do behave differently when allowed more space than the 4 sq ft per chicken indoor/10 sq ft per chicken outdoors that is often quoted on this site.
(A square meter is approximately 10.6 sq ft, fwiw)
Good luck, have fun,
Pat
